Quantification of dynamic cerebral autoregulation: welcome to the jungle!

Directional sensitivity analysis of the cerebral pressure-flow relationship during normothermia and moderate hyperthermia

Dynamic cerebral autoregulation (dCA) reacts differently when mean arterial pressure (MAP) increases versus decreases (i.e., directional sensitivity). Although heat stress alters dCA, its influence on directional sensitivity remains unclear. This analysis investigated the impact of moderate hyperthermia on the directional sensitivity in the cerebral pressure-flow relationship. Ten healthy participants (7 males; age: 37 ± 12 yr; body mass: 75 ± 9 kg) underwent 6 min of oscillatory lower body negative pressure (OLBNP) to induce large MAP fluctuations at 0.03 and 0.10 Hz under normothermic and moderately hyperthermic conditions (+1.0°C increase in core temperature) induced via a water-perfused suit. We calculated changes in middle cerebral artery mean blood velocity (MCAv) per alterations to MAP to compute absolute and relative ratios adjusted for time intervals during each OLBNP-induced MAP increase (ΔMCAvT/[Formula: see text]; %MCAvT/[Formula: see text]) and decrease (ΔMCAvT/[Formula: see text]; %MCAvT/[Formula: see text]). Thereafter, we compared average absolute and relative ratios. There was no main effect of MAP direction on ΔMCAvT/ΔMAPT or %MCAvT/%MAPT during either 0.03 Hz (P = 0.291, P = 0.281) or 0.10 Hz (P = 0.295, P = 0.178) OLBNP. Regardless of MAP direction, ΔMCAvT/[Formula: see text] (0.65 ± 0.17 vs. 0.84 ± 0.22 cm·s-1·mmHg-1), ΔMCAvT/[Formula: see text] (0.70 ± 0.15 vs. 0.85 ± 0.18 cm·s-1·mmHg-1) (thermal state: P = 0.009), %MCAvT/[Formula: see text] (0.92 ± 0.22 vs. 1.33 ± 0.60), and %MCAvT/[Formula: see text] (1.01 ± 0.27 vs. 1.30 ± 0.51) (thermal state: P = 0.001) were lower in hyperthermia at 0.03-Hz OLBNP. Regardless of thermal states, these findings suggest an absence of dCA directional sensitivity. Reduced directional sensitivity metrics during hyperthermia may indicate more efficient dCA at very low frequency.NEW & NOTEWORTHY Recent evidence highlights the importance of considering directional sensitivity in dynamic cerebral autoregulation. The current analysis found no directional sensitivity in the cerebral pressure-flow relationship during 0.03- and 0.10-Hz oscillatory lower body negative pressure in normothermia or moderate hyperthermia in healthy participants. However, reduced directional sensitivity metrics during moderate hyperthermia suggest that dynamic cerebral autoregulation may become more efficient under moderate heat stress.

Impact of successive sets of high-intensity leg press on cerebral hemodynamics across menstrual cycle phases

This study examined how successive sets of high-intensity leg press (LP) resistance exercise impact the cerebral pressure-flow relationship in untrained males and eumenorrheic females not taking oral contraceptives and assessed how the menstrual cycle (MC) phase influences the cerebral pressure-flow relationship and cerebral hemodynamics (middle cerebral artery blood velocity, MCAv; via transcranial Doppler ultrasound) during and after LP exercise in females. Young adults (11M;11F) performed three sets of leg-press exercises at 90% of their one-repetition maximum. Data from males and females in the early follicular phase were pooled together. Directional sensitivity of the cerebral pressure-flow relationship was calculated as the ratio of the rate of changes in MCAv and mean arterial pressure (MAP) (ΔMCAvT/ΔMAPT) per transition between eccentric and concentric muscular contractions during each repetition of LP exercise. ΔMCAvT/ΔMAPT was higher during concentric than eccentric phases (P < 0.001) in males and early follicular (EF) phase in females. There were no effects of successive leg press sets on any systemic or cerebral hemodynamic measures. The MC phase affected directional sensitivity and cerebral hemodynamics, with greater responses in the mid-luteal (ML) phase than the EF phase. We observed a MAP direction by MC phase interaction on relative directional sensitivity, with greater sensitivity during concentric contractions in the ML phase (P = 0.02). Our results suggest that successive sets of LP exercises do not impact the cerebral pressure-flow relationship or cerebral hemodynamics during and immediately following LP exercise. The MC phase appears to influence the cerebral pressure-flow relationship and cerebral hemodynamics both during and following LP exercise, mediated by vasoprotective effects of increased estrogen concentration in the ML phase compared with the EF phase.NEW & NOTEWORTHY Successive sets of high-intensity bilateral leg press exercises do not appear to affect cerebral or systemic hemodynamic measures, given adequate recovery time. The menstrual cycle phase impacts the directional sensitivity of the cerebral pressure-flow relationship during high-intensity bilateral leg press exercises. During high-intensity bilateral leg press exercises, the cerebrovasculature appears to be more pressure passive in the mid-luteal phase of the menstrual cycle.

New evidence for baroreflex and respiratory chemoreflex-mediated cerebral sympathetic activation in humans

The brain is highly innervated by sympathetic nerve fibers; however, their physiological purpose is poorly understood. We hypothesized that unilateral cerebral norepinephrine (NE) spillover, an index of cerebral sympathetic nerve activity (SNA), would be elevated when engaging the baroreflex [via lower-body negative pressure (LBNP; -20 and -40 Torr)] and respiratory chemoreflexes [via carbon dioxide (CO2) administration (+8 Torr)], independently and in combination. Twelve young and healthy participants (five females) underwent simultaneous blood sampling from the right radial artery and internal jugular vein. Tritiated NE was infused through the participants' right forearm vein. Right internal jugular vein and internal carotid artery blood flow were measured using duplex ultrasound. Unilateral cerebral NE spillover remained unchanged when only LBNP was applied (P = 0.29) but increased with hypercapnia (P = 0.035) and -40 Torr LBNP + CO2 (P < 0.01). There were no changes in total NE spillover during the LBNP and LBNP + CO2 trials (both P = 0.66), nor during CO2 alone (P = 0.13). No correlations were present between the increase in unilateral cerebral NE spillover during -40 Torr LBNP + CO2 and reductions in internal carotid artery blood flow (P = 0.56). These results indicate that baroreflex and respiratory chemoreflex stressors elevate cerebral SNA; however, the observed cerebral sympathetic activation has no impact on blood flow regulation in the internal carotid artery.NEW & NOTEWORTHY The results of the current study suggest that baroreflex and respiratory chemoreflex stressors elevate cerebral sympathetic nervous activity, quantified using the brain norepinephrine spillover method. However, the observed cerebral sympathetic activation has no impact on blood flow regulation in the internal carotid artery.

Cerebral oximetry index indicates delirium or stroke after carotid endarterectomy: An observational study

BACKGROUNDS

The cerebral oximetry index (COx) uses near-infrared spectroscopy to estimate cerebral autoregulation during cardiac surgery. However, the relationship between intraoperative loss of cerebral autoregulation and postoperative delirium or stroke remains unclear in patients recovering from carotid endarterectomy (CEA).

METHODS

Our prospective observational cohort study enrolled patients scheduled for CEA. COx was estimated as the coefficient of a continuous, moving Spearman correlation between mean arterial pressure and cerebral oxygen saturation. A receiver operating characteristics curve with Youden's index identified the optimal COx threshold for predicting a composite of postoperative delirium or new-onset overt stroke.

RESULTS

One hundred and forty patients scheduled for CEA were enrolled. The incidence of delirium was 10.7 % (15/140) and the incidence of stroke was 3.6 % (5/140), including 1 patient who had both. The cumulative anesthesia time when COx exceeded 0.3 was longer in patients with complications than those without. When COx > 0.6, the corresponding predictive ability was AUC = 0.69, Youden index = 0.61, P = 0.0003, with a positive predictive value of 100 %. In the post hoc subgroup analyses, before clamping, the greatest increase in the risk was observed when COx > 0.7 for 20 min (Odds ratio = 3.10, 95 % CI 2.20, 3.78). In contrast, COx was not predictive during clamping. After clamping, the optimal COx threshold was 0.4 (AUC = 0.85, Youden index = 0.82, P < 0.0001), with the positive predictive value being 100 %.

CONCLUSIONS

COx is a promising metric for predicting postoperative delirium or new-onset overt stroke in patients having CEA. The optimal COx threshold was 0.7 in the pre-clamping phase and 0.4 in the post-clamping phase.

A multimodal neuroimaging study of cerebrovascular regulation: protocols and insights of combining electroencephalography, functional near-infrared spectroscopy, transcranial Doppler ultrasound, and physiological parameters

Objective. The current paper describes the creation of a simultaneous trimodal neuroimaging protocol. The authors detail their methodological design for a subsequent large-scale study, demonstrate the ability to obtain the expected physiologically induced responses across cerebrovascular domains, and describe the pitfalls experienced when developing this approach.Approach. Electroencephalography (EEG), functional near-infrared spectroscopy (fNIRS), and transcranial Doppler ultrasound (TCD) were combined to provide an assessment of neuronal activity, microvascular oxygenation, and upstream artery velocity, respectively. Real-time blood pressure, capnography, and heart rate were quantified to control for the known confounding influence of cardiorespiratory variables. The EEG-fNIRS-TCD protocol was attached to a 21 year-old male who completed neurovascular coupling/functional hyperemia (finger tapping and 'Where's Waldo/Wally?'), dynamic cerebral autoregulation (squat-stand maneuvers), and cerebrovascular reactivity tasks (end-tidal clamping during hypocapnia/hypercapnia).Main results. In a pilot participant, the Waldo task produced robust hemodynamic responses within the occipital microvasculature and the posterior cerebral artery. A ∼90% decrease in alpha band power was seen in the occipital cortical region compared between the eyes closed and eyes opened protocol, compared to the frontal, central, and parietal regions (∼80% reduction). A modest increase in motor oxygenated hemoglobin was seen during the finger tapping task, with a harmonious alpha decrease of ∼15% across all cortical regions. No change in the middle or posterior cerebral arteries were noted during finger tapping. During cerebral autoregulatory challenges, sinusoidal oscillations were produced in hemodynamics at 0.05 and 0.10 Hz, while a decrease and increase in TCD and fNIRS metrics were elicited during hypocapnia and hypercapnia protocols, respectively.Significance. All neuroimaging modalities have their inherent limitations; however, these can be minimized by employing multimodal neuroimaging approaches. This EEG-fNIRS-TCD protocol enables a comprehensive assessment of cerebrovascular regulation across the association between electrical activity and cerebral hemodynamics during tasks with a mild degree of body and/or head movement.

Subcomponent analysis of the directional sensitivity of dynamic cerebral autoregulation

The origin of the directional sensitivity (DS) of dynamic cerebral autoregulation (dCA) is not known. In 140 healthy participants (67 male, 27.5 ± 6.1 yr old), middle cerebral artery velocity (MCAv, transcranial Doppler), arterial blood pressure (ABP, Finometer), and end-tidal CO2 (EtCO2, capnography) were recorded at rest. Critical closing pressure (CrCP) and resistance-area product (RAP) were obtained for each cardiac cycle, as well as mean MCAv and ABP (MAP). The integrated positive and negative derivatives of MAP (MAP+D and MAP-D, respectively) were used as simultaneous inputs to an autoregressive moving average model to generate two distinct MCAv step responses. Similar models allowed the estimation of corresponding MAP-CrCP and MAP-RAP responses to step changes in MAP+D and MAP-D. The strength of DS (ΔDS) was expressed by the difference in mean values of the step responses for the time interval 12-18 s. ΔDS was significant for MCAv (8.5 ± 46.9% vs. 26.7 ± 42.0%, P < 0.001) and RAP (-93.9 ± 48.1 vs. -74.5 ± 43.0%, P < 0.001), respectively, for MAP+D and MAP-D inputs, but not for CrCP (2.2 ± 48.1% vs. 0.72 ± 42.9%, P = 0.76). Compared with males, female participants had higher MCAv (63.9 ± 15.6 cm/s vs. 55.4 ± 12.9 cm/s, P < 0.001) but lower EtCO2 (P < 0.001) and RAP (P = 0.015). Sex did not influence ΔDS for any of the three-step responses. The presence of directional sensitivity in the RAP, but not in the CrCP transfer function, suggests that the origin could be solely myogenic, without metabolic involvement.NEW & NOTEWORTHY The directional sensitivity of the cerebral blood velocity response to a sudden change in mean arterial blood pressure (MAP) is mediated by the resistance-area product, without involvement from the cerebral critical closing pressure. The reduced amplitude of MAP spontaneous fluctuations at rest suggests that it is less likely that directional sensitivity has origins in the sympathetic control of cerebral blood vessels, thus generating the need to consider other alternatives.

Physiological influences on neurovascular coupling: A systematic review of multimodal imaging approaches and recommendations for future study designs

In this review, we have amalgamated the literature, taking a multimodal neuroimaging approach to quantify the relationship between neuronal firing and haemodynamics during a task paradigm (i.e., neurovascular coupling response), while considering confounding physiological influences. Original research articles that used concurrent neuronal and haemodynamic quantification in humans (n ≥ 10) during a task paradigm were included from PubMed, Scopus, Web of Science, EMBASE and PsychINFO. Articles published before 31 July 2023 were considered for eligibility. Rapid screening was completed by the first author. Two authors completed the title/abstract and full-text screening. Article quality was assessed using a modified version of the National Institutes of Health Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. A total of 364 articles were included following title/abstract and full-text screening. The most common combination was EEG/functional MRI (68.7%), with cognitive (48.1%) and visual (27.5%) tasks being the most common. The majority of studies displayed an absence/minimal control of blood pressure, arterial gas concentrations and/or heart rate (92.9%), and only 1.3% monitored these factors. A minority of studies restricted or collected data pertaining to caffeine (7.4%), exercise (0.8%), food (0.5%), nicotine (2.7%), the menstrual cycle (0.3%) or cardiorespiratory fitness levels (0.5%). The cerebrovasculature is sensitive to numerous factors; thus, to understand the neurovascular coupling response fully, better control for confounding physiological influences of blood pressure and respiratory metrics is imperative during study-design formulation. Moreover, further work should continue to examine sex-based differences, the influence of sex steroid hormone concentrations and cardiorespiratory fitness.

Cardiac and respiratory activities induce temporal changes in cerebral blood volume, balanced by a mirror CSF volume displacement in the spinal canal

Understanding cerebrospinal fluid (CSF) dynamics is crucial for elucidating the pathogenesis and diagnosis of neurodegenerative diseases. The primary mechanisms driving CSF oscillations remain a topic of debate. This study investigates whether cerebral blood volume displacement (CBV), modulated by breathing and cardiac activity, is the predominant drivers of CSF oscillations. We examined 12 healthy volunteers (aged 20-34 years) using a clinical 3T MRI scanner to quantify cerebral blood flow at the intracranial level and CSF flow at the C2-C3 spinal level under free and deep breathing conditions, utilizing real-time phase-contrast sequences. We then obtained CBV and CSF volume displacement (CSFV) curves by integrating the flow rate signals. Cardiac and respiratory signals were recorded during acquisition to reconstruct cardiac-driven and breath-driven CBV and CSFV curves. During deep breathing, compared to free breathing, the total cerebral arterial flow rate decreased by 29 % (from 12.5 ml/s to 8.8 ml/s), and the duration of the cardiac cycle period shortened by 15 % (0.90 s to 0.77 s), leading to reductions of 37 % and 23 % in cardiac-driven CBV and CSFV amplitudes, respectively. Conversely, breath-driven CBV and CSFV amplitudes increased substantially by 207 % and 326 %, respectively. Notably, during free breathing, cardiac-driven CBV and CSFV were significantly greater than their breath-driven counterparts; however, during deep breathing, the amplitudes of cardiac-driven and breath-driven CBV and CSFV did not differ significantly. CBV and CSFV curves demonstrated strong coupled inverse oscillation under both breathing conditions, with consistent CSF inflow toward the intracranial compartment during inspiration. This study quantifies the contributions of cardiac and breathing activities to CBV and CSFV under varying breathing patterns, confirming that CBV changes, driven by cardiac and respiratory activities, are strongly inversely coupled with CSF oscillations. These findings enhance our understanding of CSF circulation mechanisms and offer potential diagnostic implications for neurodegenerative diseases.

Challenging dynamic cerebral autoregulation across the physiological CO2 spectrum: Influence of biological sex and cardiac cycle

This study applied alterations in partial pressure of end-tidal carbon dioxide (P ETC O 2 ${{P}_{{\mathrm{ETC}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ) to challenge dynamic cerebral autoregulation (dCA) responses across the cardiac cycle in both biological sexes. A total of 20 participants (10 females and 10 males; aged 19-34 years) performed 4-min bouts of repeated squat-stand manoeuvres (SSMs) at 0.05 and 0.10 Hz (randomized orders) withP ETC O 2 ${{P}_{{\mathrm{ETC}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ clamped at ∼40 mmHg. The protocol was repeated for hypercapnic (∼55 mmHg) and hypocapnic (∼20 mmHg) conditions. Middle cerebral artery (MCA) and posterior cerebral artery (PCA) were insonated via transcranial Doppler ultrasound. Dynamic end-tidal forcing clampedP ETC O 2 ${{P}_{{\mathrm{ETC}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ , and finger photoplethysmography quantified beat-to-beat changes in blood pressure. Linear regressions were performed for transfer function analysis metrics including power spectrum densities, coherence, phase, gain and normalized gain (nGain) with adjustment for sex. During hypercapnic conditions, phase metrics were reduced from eucapnic levels (all P < 0.009), while phase increased during the hypocapnic stage during both 0.05 and 0.10 Hz SSMs (all P < 0.037). Sex differences were present with females displaying greater gain and nGain systole metrics during 0.10 Hz SSMs (all P < 0.041). AcrossP ETC O 2 ${{P}_{{\mathrm{ETC}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ stages, females displayed reduced buffering against systolic aspects of the cardiac cycle and augmented gain. Sex-related variances in dCA could explain sex differences in the occurrence of clinical conditions such as orthostatic intolerance and stroke, though the effect of fluctuating sex hormones and contraceptive use on dCA metrics is not yet understood.

Individualized autoregulation-guided arterial blood pressure management in neurocritical care

Cerebral autoregulation (CA) is the physiological process by which cerebral blood flow is maintained during fluctuations in arterial blood pressure (ABP). There are various validated methods to measure CA, either invasively, with intracranial pressure or brain tissue oxygenation monitors, or noninvasively, with transcranial Doppler ultrasound or near-infrared spectroscopy. Utilizing these monitors, researchers have been able to discern CA patterns in several pathological states, such as but not limited to acute ischemic stroke, spontaneous intracranial hemorrhage, aneurysmal subarachnoid hemorrhage, sepsis, and post-cardiac arrest, and they have found CA to be altered in these patients. CA disturbances predispose patients suffering from these ailments to worse outcomes. Much focus has been placed on CA monitoring in these populations, with an emphasis on arterial blood pressure optimization. Many guidelines recommend universal static ABP targets; however, in patients with altered CA, these targets may make them susceptible to hypoperfusion and further neurological injury. Based on this observation, there has been much investigation on individualized ABP goals and their effect on clinical outcomes. The scope of this review includes (1) a summary of the physiology of CA in healthy adults; (2) a review of the evidence on CA monitoring in healthy individuals; (3) a summary of CA changes and its effect on outcomes in various diseased states including acute ischemic stroke, spontaneous intracranial hemorrhage, aneurysmal subarachnoid hemorrhage, sepsis and meningitis, post-cardiac arrest, hypoxic-ischemic encephalopathy, surgery, and moyamoya disease; and (4) a review of the current evidence on individualized ABP changes in various patient populations.

Examining the upper frequency limit of dynamic cerebral autoregulation: Considerations across the cardiac cycle during eucapnia

There are differences within the literature regarding the upper frequency cut-off point of the dynamic cerebral autoregulation (CA) high-pass filter. The projection pursuit regression approach has demonstrated that the upper frequency limit is ∼0.07 Hz, whereas another approach [transfer function analysis (TFA) phase approaching zero] indicated a theoretical upper frequency limit for the high-pass filter of 0.24 Hz. We investigated how these limits accurately represent the CA upper frequency limit, in addition to extending earlier findings with respect to biological sexes and across the cardiac cycle. Sixteen participants (nine females and seven males) performed repeated squat-stand manoeuvres at frequencies of 0.05, 0.10, 0.15, 0.20 and 0.25 Hz, with insonation of the middle and posterior cerebral arteries. Linear regression modelling with adjustment for sex and order of squat completion was used to compared TFA gain and phase with 0.25 Hz (above the theoretical limit of CA). The upper frequency limit of CA with TFA gain was within the range of 0.05-0.10 Hz, whereas TFA phase was within the range of 0.20-0.25 Hz, and consistent between vessels, between sexes and across the cardiac cycle. Females displayed greater middle cerebral artery gain compared with males (all P < 0.047), and no phase differences were present (all P > 0.072). Although sex-specific differences were present for specific TFA metrics at a given frequency, the upper frequency limit of autoregulation was similar between cerebral conduit vessels, cardiac cycle phase and biological sex. Future work is warranted to determine whether an upper frequency limit exists with respect to hysteresis analyses.

Quantifying neurovascular coupling through a concurrent assessment of arterial, capillary, and neuronal activation in humans: A multimodal EEG-fNIRS-TCD investigation

BACKGROUND

This study explored a novel multimodal neuroimaging approach to assess neurovascular coupling (NVC) in humans using electroencephalography (EEG), functional near-infrared spectroscopy (fNIRS), and transcranial Doppler ultrasound (TCD).

METHODS

Fifteen participants (nine females; age 19-32) completed concurrent EEG-fNIRS-TCD imaging during motor (finger tapping) and visual ("Where's Waldo?") tasks, with synchronized monitoring of blood pressure, capnography, and heart rate. fNIRS assessed microvascular oxygenation within the frontal, motor, parietal, and occipital cortices, while the middle and posterior cerebral arteries (MCA/PCA) were insonated using TCD. A 16-channel EEG set-up was placed according to the 10-20 system. Wilcoxon signed-rank tests were used to compare physiological responses between the active and resting phases of the tasks, while cross-correlations with zero legs compared cerebral and systemic hemodynamic responses across both tasks.

RESULTS

Time-frequency analysis demonstrated a reduction in alpha and low beta band power in electrodes C3/C4 during finger tapping (p<0.045) and all electrodes during the Waldo task (all p<0.001). During Waldo, cross-correlation analysis demonstrated the change in oxygenated hemoglobin and cerebral blood velocity had a moderate-to-strong negative correlation with systemic physiological influences, highlighting the measured change resulted from neuronal input. Deoxygenated hemoglobin displayed the greatest negative cross-correlation with the MCA/PCA within the motor cortices and visual during the motor and visual tasks, respectively (range:0.54, -0.82).

CONCLUSIONS

This investigation demonstrated the feasibility of the proposed EEG-fNIRS-TCD response to comprehensively assess the NVC response within human, specifically quantifying the real-time temporal synchrony between neuronal activation (EEG), microvascular oxygenation changes (fNIRS), and conduit artery velocity alterations (TCD).

Data-based modeling of cerebral hemodynamics quantifies impairment of cerebral blood flow regulation in type-2 diabetes

We studied the regulation dynamics of cerebral blood velocity (CBv) at middle cerebral arteries (MCA) in response to spontaneous changes of arterial blood pressure (ABP), termed dynamic cerebral autoregulation (dCA), and end-tidal CO2 as proxy for blood CO2 tension, termed dynamic vasomotor reactivity (DVR), by analyzing time-series data collected at supine rest from 36 patients with Type-2 Diabetes Mellitus (T2DM) and 22 age/sex-matched non-diabetic controls without arterial hypertension. Our analysis employed a robust dynamic modeling methodology that utilizes Principal Dynamic Modes (PDM) to estimate subject-specific dynamic transformations of spontaneous changes in ABP and end-tidal CO2 (viewed as two "inputs") into changes of CBv at MCA measured via Transcranial Doppler ultrasound (viewed as the "output"). The quantitative results of PDM analysis indicate significant alterations in T2DM of both DVR and dCA in terms of two specific PDM contributions that rise to significance (p < 0.05). Our results further suggest that the observed DVR and dCA alterations may be due to reduction of cholinergic activity (based on previously published results from cholinergic blockade data) that may disturb the sympatho-vagal balance in T2DM. Combination of these two model-based "physio-markers" differentiated T2DM patients from controls (p = 0.0007), indicating diabetes-related alteration of cerebrovascular regulation, with possible diagnostic implications.

Directional sensitivity of the cerebral pressure-flow relationship during forced oscillations induced by oscillatory lower body negative pressure

A directional sensitivity of the cerebral pressure-flow relationship has been described using repeated squat-stands. Oscillatory lower body negative pressure (OLBNP) is a reproducible method to characterize dynamic cerebral autoregulation (dCA). It could represent a safer method to examine the directional sensitivity of the cerebral pressure-flow relationship within clinical populations and/or during pharmaceutical administration. Therefore, examining the cerebral pressure-flow directional sensitivity during an OLBNP-induced cyclic physiological stress is crucial. We calculated changes in middle cerebral artery mean blood velocity (MCAv) per alterations to mean arterial pressure (MAP) to compute ratios adjusted for time intervals (ΔMCAvT/ΔMAPT) with respect to the minimum-to-maximum MCAv and MAP, for each OLBNP transition (0 to -90 Torr), during 0.05 Hz and 0.10 Hz OLBNP. We then compared averaged ΔMCAvT/ΔMAPT during OLBNP-induced MAP increases (INC) (ΔMCAvT/Δ MAP T INC ) and decreases (DEC) (ΔMCAvT/Δ MAP T DEC ). Nineteen healthy participants [9 females; 30 ± 6 years] were included. There were no differences in ΔMCAvT/ΔMAPT between INC and DEC at 0.05 Hz. ΔMCAvT/Δ MAP T INC (1.06 ± 0.35 vs. 1.33 ± 0.60 cm⋅s-1/mmHg; p = 0.0076) was lower than ΔMCAvT/Δ MAP T DEC at 0.10 Hz. These results support OLBNP as a model to evaluate the directional sensitivity of the cerebral pressure-flow relationship.

Time-domain methods for quantifying dynamic cerebral blood flow autoregulation: Review and recommendations. A white paper from the Cerebrovascular Research Network (CARNet)

Cerebral Autoregulation (CA) is an important physiological mechanism stabilizing cerebral blood flow (CBF) in response to changes in cerebral perfusion pressure (CPP). By maintaining an adequate, relatively constant supply of blood flow, CA plays a critical role in brain function. Quantifying CA under different physiological and pathological states is crucial for understanding its implications. This knowledge may serve as a foundation for informed clinical decision-making, particularly in cases where CA may become impaired. The quantification of CA functionality typically involves constructing models that capture the relationship between CPP (or arterial blood pressure) and experimental measures of CBF. Besides describing normal CA function, these models provide a means to detect possible deviations from the latter. In this context, a recent white paper from the Cerebrovascular Research Network focused on Transfer Function Analysis (TFA), which obtains frequency domain estimates of dynamic CA. In the present paper, we consider the use of time-domain techniques as an alternative approach. Due to their increased flexibility, time-domain methods enable the mitigation of measurement/physiological noise and the incorporation of nonlinearities and time variations in CA dynamics. Here, we provide practical recommendations and guidelines to support researchers and clinicians in effectively utilizing these techniques to study CA.

A systematic review, meta-analysis and meta-regression amalgamating the driven approaches used to quantify dynamic cerebral autoregulation

Numerous driven techniques have been utilized to assess dynamic cerebral autoregulation (dCA) in healthy and clinical populations. The current review aimed to amalgamate this literature and provide recommendations to create greater standardization for future research. The PubMed database was searched with inclusion criteria consisting of original research articles using driven dCA assessments in humans. Risk of bias were completed using Scottish Intercollegiate Guidelines Network and Methodological Index for Non-Randomized Studies. Meta-analyses were conducted for coherence, phase, and gain metrics at 0.05 and 0.10 Hz using deep-breathing, oscillatory lower body negative pressure (OLBNP), sit-to-stand maneuvers, and squat-stand maneuvers. A total of 113 studies were included, with 40 of these incorporating clinical populations. A total of 4126 participants were identified, with younger adults (18-40 years) being the most studied population. The most common techniques were squat-stands (n = 43), deep-breathing (n = 25), OLBNP (n = 20), and sit-to-stands (n = 16). Pooled coherence point estimates were: OLBNP 0.70 (95%CI:0.59-0.82), sit-to-stands 0.87 (95%CI:0.79-0.95), and squat-stands 0.98 (95%CI:0.98-0.99) at 0.05 Hz; and deep-breathing 0.90 (95%CI:0.81-0.99); OLBNP 0.67 (95%CI:0.44-0.90); and squat-stands 0.99 (95%CI:0.99-0.99) at 0.10 Hz. This review summarizes clinical findings, discusses the pros/cons of the 11 unique driven techniques included, and provides recommendations for future investigations into the unique physiological intricacies of dCA.

Describing the Response of Cerebral Autoregulation to Postural Challenge via State Space Correspondence Methods

Postural stimulus influences the cerebral autoregulation (CA), but it remains to be elucidated whether its impact is transient. Two nonlinear state space correspondence (SSC) methods, based on cross-predictability (CP) and cloud size ratio (CSR), were exploited to describe the CA from spontaneous variability of mean arterial pressure (MAP) and mean cerebral blood velocity (MCBv) during sustained postural challenge. We continuously recorded MAP and MCBv in 12 healthy subjects (age: 27 ± 8 yrs; 5 males) at rest in supine position and during head-up tilt (HUT) at 60° prolonged for 20 minutes after the onset of the orthostatic challenge. We found that: i) markers of MCBv-MAP association computed by CP and CSR techniques are significantly associated but they feature evident differences especially in the early phase of the HUT; ii) both the methods detect an increase of the degree of association from MAP to MCBv during the late phase of HUT. We conclude that, even though the two SSC methods cannot be considered interchangeable, both techniques indicate that HUT affects the CA, and its modifications are not limited to the early phase of HUT.

Letter to the editor: Deriving transfer function analysis metrics from driven methods

Driven and spontaneous methods have been used to quantify the cerebral pressure-flow relationship via transfer function analysis (TFA). Commonly, TFA derived estimates are assessed using band averages within the very-low (0.02-0.07 Hz) and low (0.07-0.20 Hz) frequency during spontaneous oscillations but are quantified at frequencies of interest where blood pressure oscillations are driven (e.g., 0.05 and/or 0.10 Hz). Driven estimates more closely resemble the autoregulatory challenges individuals experience on a daily basis, while also eliciting higher levels of reliability. While driven estimates with point-estimates are not feasible for all clinical populations, these approaches increase the ability to understand pathophysiological changes.

Lower dynamic cerebral autoregulation following acute bout of low-volume high-intensity interval exercise in chronic stroke compared to healthy adults

Fluctuating arterial blood pressure during high-intensity interval exercise (HIIE) may challenge dynamic cerebral autoregulation (dCA), specifically after stroke after an injury to the cerebrovasculature. We hypothesized that dCA would be attenuated at rest and during a sit-to-stand transition immediately after and 30 min after HIIE in individuals poststroke compared with age- and sex-matched control subjects (CON). HIIE switched every minute between 70% and 10% estimated maximal watts for 10 min. Mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) were recorded. dCA was quantified during spontaneous fluctuations in MAP and MCAv via transfer function analysis. For sit-to-stand, time delay before an increase in cerebrovascular conductance index (CVCi = MCAv/MAP), rate of regulation, and % change in MCAv and MAP were measured. Twenty-two individuals poststroke (age 60 ± 12 yr, 31 ± 16 mo) and twenty-four CON (age 60 ± 13 yr) completed the study. Very low frequency (VLF) gain (P = 0.02, η2 = 0.18) and normalized gain (P = 0.01, η2 = 0.43) had a group × time interaction, with CON improving after HIIE whereas individuals poststroke did not. Individuals poststroke had lower VLF phase (P = 0.03, η2 = 0.22) after HIIE compared with CON. We found no differences in the sit-to-stand measurement of dCA. Our study showed lower dCA during spontaneous fluctuations in MCAv and MAP following HIIE in individuals poststroke compared with CON, whereas the sit-to-stand response was maintained.NEW & NOTEWORTHY This study provides novel insights into poststroke dynamic cerebral autoregulation (dCA) following an acute bout of high-intensity interval exercise (HIIE). In people after stroke, dCA appears attenuated during spontaneous fluctuations in mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) following HIIE. However, the dCA response during a single sit-to-stand transition after HIIE showed no significant difference from controls. These findings suggest that HIIE may temporarily challenge dCA after exercise in individuals with stroke.

A Quantitative Assessment of Cerebral Hemodynamic Perturbations Associated with Long R-R Intervals in Atrial Fibrillation: A Pilot-Case-Based Experience

Background and Objectives: Atrial fibrillation (AF) results in systemic hemodynamic perturbations which impact cerebral circulation, possibly contributing to the development of dementia. However, evidence documenting effects in cerebral perfusion is scarce. The aim of this study is to provide a quantitative characterization of the magnitude and time course of the cerebral hemodynamic response to the short hypotensive events associated with long R-R intervals, as detected by near-infrared spectroscopy (NIRS). Materials and Methods: Cerebral NIRS signals and arterial blood pressure were continuously recorded along with an electrocardiogram in twelve patients with AF undergoing elective electrical cardioversion (ECV). The top 0.5-2.5% longest R-R intervals during AF were identified in each patient and used as triggers to carry out the triggered averaging of hemodynamic signals. The average curves were then characterized in terms of the latency, magnitude, and duration of the observed effects, and the possible occurrence of an overshoot was also investigated. Results: The triggered averages revealed that long R-R intervals produced a significant drop in diastolic blood pressure (-13.7 ± 6.1 mmHg) associated with an immediate drop in cerebral blood volume (THI: -0.92 ± 0.46%, lasting 1.9 ± 0.8 s), followed by a longer-lasting decrease in cerebral oxygenation (TOI: -0.79 ± 0.37%, lasting 5.2 ± 0.9 s, p < 0.01). The recovery of the TOI was generally followed by an overshoot (+1.06 ± 0.12%). These effects were progressively attenuated in response to R-R intervals of a shorter duration. Conclusions: Long R-R intervals cause a detectable and consistent cerebral hemodynamic response which concerns both cerebral blood volume and oxygenation and outlasts the duration of the systemic perturbation. These effects are compatible with the activation of dynamic autoregulatory mechanisms in response to the hypotensive stimulus.